1. Field of the Invention
The present invention relates to a multi-stage hydraulic machine and to a control method for a multi-stage hydraulic machine having a plurality of stages from the highest-pressure stage to the lowest-pressure stage. More particularly, the present invention relates to a multi-stage hydraulic machine and a control method for a multi-stage hydraulic machine in which the runner chambers of adjacent stages communicate with each other through a return channel and at least the highest-pressure stage is provided with a movable wicket gate. The control method is applicable to the period of time during which the mode of operation is shifted from idling operation before power generating or the pumping, to the power generating operation or the pumping operation.
2. Description of the Prior Art
In single-stage hydraulic machines used in pumped storage power plants, highly pressurized air is in general fed by a water level depressor to a point above the water level at the upper part of a suction pipe in order to reduce the torque for driving a runner during the turbine starting operation or the pump starting operation. The air thus fed causes the water level to go down so that the runner is operated in air. Thereafter, the air fed into the channel path is exhausted and the water level goes up and the flow path is filled with water. In this manner, the hydraulic machine is changed in mode of operation to a predetermined turbine generating or pumping operation.
In a multi-stage hydraulic machine having a plurality of stages, the runner chambers of adjacent stages communicate with each other through a return channel. Therefore, the multi-stage hydraulic machine has a complicated channel system. Thus, various difficulties are found in the air exhausting operation when an idling operation is shifted to the power generating operation or the pumping operation.
Particularly in the case of a multi-stage hydraulic machine provided with movable wicket gates in the highest-pressure stage, in order to safely control the driven condition during this transition phase, the channels of the respective stages from the highest-pressure stage to the lowest-pressure stage are in constant communication with each other.
When the mode of operation is shifted from idling operation to power generating or pumping, water is fed from the runner chamber of the lowest-pressure stage to the runner chamber of the highest-pressure stage. Consequently, highly pressurized water driven by the runner of the lowest-pressure stage is rapidly fed into the highest-pressure stage so that the air in the highest-pressure stage is compressed, and at the same time the runner chamber in the highest-pressure stage is rapidly filled by the highly pressurized water.
As a result, the water pressure of the runner chamber in the highest-pressure stage is quickly raised and the driving torque for the runner is rapidly increased from a torque necessary for idling operation to a torque necessary for an underwater shut-off state in which the runner chamber is filled with water. Thus such a multi-stage hydraulic machine receives a shock force. Moreover, an impulsive twist force is applied to a shaft of the motor which drives the runner of the hydraulic machine, and consequently the electric power regulation in the electric power system to which the motor is connected is difficult.
Therefore, the conventional multi-stage hydraulic machine is a relatively small capacity type, and it uses an underwater starting system in which the hydraulic machine is started in the state that each runner chamber is filled with water, without depressing the water level even when the pumping operation is started, and the machine runners are then accelerated to a rated speed in this state.
In a large capacity type multi-stage hydraulic machine in which the adoption of such an underwater starting system would be difficult, the safe and smooth shifting from an idling operation to a power generating or pumping operation is, however, an important technical problem to be solved.